Add Content-Type to REST service documentation

The reference guide does not explicitly indicate the need for a Content-Type: application/json header in HTTP POST requests to the REST server. If omitted, this can result in a failure with a misleading error message. Enhance the documentation to include the Content-Type header in the examples, and/or include an actual example of using a tool like wget or curl to submit a request. Generally speaking, make an explicit statement about the Content-Type requirement.